Brand loyalty is only a question of money. If you’ve enough cash to swap manufacturers and replace your kit entirely, then of course that’s an option to consider. I shoot Canon, have done for 20 years so I can explain that range in depth.
Canon make two lines of pro cameras with each line having a bias to either sports/action/journo type shooting (50D, 1D MarkIII) where speed and accuracy are the most important factors, or studio/glamour/landscape shooting where the subject isn’t dashing around so much and therefore quality can come to the front at the expense of focus / processing speed (5D Mark1 and II, 1Ds Mark III).
Now, these cameras are at least a grand a pop so we are talking serious cash for hobby-shooters. Below this level Canon only have one range where of course marketable features are the design ethos, essentially megapixels. This range is the 450d, 500d etc - all of which are fine cameras capable of taking a great image but they have plastic bodies (don’t drop them) and much more limited AF capabilities and don’t process images with as much care as the pro range (megapixels are only the starting point for a good picture).
Since I shoot motorsports far more than anything else these days my camera choice had to suit that hence I have a 10.2mp 1D MarkIII, but that’s backed up by a 8.2mp 20D which is about 4 years old. Comparing the two it is obvious that the 1D has a far quicker and more accurate AF system and produces a slightly sharper and more balanced image than the older 20D. However, the 20D can still track subjects pretty well and when it’s on the money I get a very pleasing image from it. The BSB galleries are a mix from both cameras and I defy anyone to tell which shots were taken on my £250 2nd hand 20D or my owned from new £2500 1D.
But that’s Canon.
As far as Nikon goes, I haven’t had too much exposure to that brand. I own a 1972 F2 Photomic but that’s not important here. Over the last 4 years I’ve shot with friends who all used Nikon - the D70s, D200 and now the D700 - and they’ve all impressed me with the final result but infuriated me with the camera design and menu layout (surely a case of getting used to it). So I have nothing against Nikon at all, their lenses were at worst level with the Canon equivalent but often slightly better quality.
I can tell you that in the BSB pressroom about 90% of the shooters use Canon simply because Nikon have always lagged a long way behind Canon in terms of AF and high ISO performance. Both are important to motorsport, high ISO especially so in this dark wet country. The D2X for example was a good camera for motorsport, but handicapped by a 6mp sensor if memory serves, and shooting at 400ISO or above screwed the results.
Now the D700 and presumably to D300 have moved the game on for Nikon, I know they’ve certainly cured the ISO issue and I imagine their AF performance is now much improved. Hell, the D3 is possibly the best DSLR out there now but (it should be for £5k) so they’ve a good lineage to work from.
From the last generation the D80 or is the the D90 were the pro-am models akin to the Canon 40 and 50D, so try second hand examples of those. Look at the D2x tool but see if you can shoot a few and check the quality.
My overall recommendation would be
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do some homework as to camera specs and 2nd hand prices
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trot along to Grays of Westminster - they are possibly the most respected Nikon retailer in London with a huge 2nd hand stock. Take a memory card, load it in different bodies and shoot the same subject to compare at home. And talk to them!
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don’t forget a decent lens is just as important.
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practice practice practice.